1. Field of the Invention
The present invention relates to a built-in motor type electric injection molding apparatus.
2. Description of the Related Art
Conventionally, in an injection molding apparatus, resin heated and melted in a heating cylinder is injected into a cavity of a die under a high pressure so that the cavity is filled with the resin. The molten resin is then cooled and hardened to obtain a molded product. The molded product is then taken out from the die after the die is opened.
The injection molding apparatus includes a die clamp apparatus and an injection apparatus. The die clamp apparatus is provided with a stationary platen and a movable platen, and the die is opened and closed by advancing and retracting the movable platen using a die clamping cylinder.
The injection apparatus includes a heating cylinder for heating and melting resin supplied from a hopper and an injection nozzle for injecting the molten resin. Further, a screw is disposed within the heating cylinder for advancing and retracting movement. The screw is advanced to inject resin and retracted to meter the resin.
An electric injection molding apparatus has been proposed in which electric motors are used to advance and retract the injection apparatus and to advance and retract the screw.
FIG. 1 is a schematic view of an injection apparatus used in a conventional electric injection molding apparatus.
In FIG. 1, numeral 2 denotes an injection apparatus, and numeral 4 denotes a frame of the injection apparatus 2. A heating cylinder 21 is fixed on the front side (left side in FIG. 1) of the frame 4, and an unillustrated injection nozzle is provided at the front end (left-side end in FIG. 1) of the heating cylinder 21. A screw 20 is disposed within the heating cylinder 21 such that the screw 20 is rotatable and axially movable. A ball screw 31 is formed at the rear end portion of the screw 20 and a spline shaft 32 is extended from the rear end of the ball screw 31.
An injection motor 34 is attached to the frame 4 such that the injection motor 34 surrounds the ball screw 31, and a ball screw nut 37 is fixed to the injection motor 34. Further, a metering motor 35 is disposed such that the metering motor 35 surrounds the spline shaft 32, and a spline nut 38 is fixed to the metering motor 35.
A numerical controller 39 is connected to the injection motor 34 and to the metering motor 35. Injection and metering are performed by selectively rotating these motors 34 and 35 by the numerical controller 39. In detail, in a metering stage, the metering motor 35 and the injection motor 34 are simultaneously rotated at the same speed, so that the spline shaft 32, the ball screw 31 and the screw 20 rotate for metering. At this time, power supplied to the injection motor 34 can be adjusted to provide a difference in rotational speed between the ball screw nut 37 and the spline nut 38, thereby retracting the screw 20. With this operation, back pressure can be controlled during metering.
In an injection stage, the injection motor 34 is rotated while the metering motor 35 is stopped, so that the ball screw 31 is advanced by rotation of the ball screw nut 37. As a result, the screw 20 is advanced to perform injection.
However, in the injection apparatus of the conventional electric injection molding apparatus, when heat generated by the metering motor 35 and the injection motor 34 is accumulated within an unillustrated drive case, the metering motor 35 and the injection motor 34 overheat, thus lowering the output ratings of the metering motor 35 and the injection motor 34. In addition, it is impossible to select optimal drive characteristics corresponding to the load of the electric injection molding apparatus in use.